The present invention relates to apparatus and methods for determining flow characteristics of a fluid, and more particularly to apparatus and methods for determining the flow rate of a polyphase fluid or a turbulent single-phase fluid by detecting pressure gradients within the fluid.
Generally speaking, apparatus and methods for determining flow characteristics of a fluid are known. For example, French Pat. No. 2,161,834 (Applicant, Societe Nationale Des Petroles D'Aquitaine) is directed to the determination of the rate of flow of phase domaines in a polyphase fluid and to the determination of the composition of the polyphase fluid. The Patent discloses the use of a Wheatstone bridge as a differential sensor, a hot-film probe being included in each of the two branches of the Wheatstone bridge. One of the two probes of each differential sensor is placed at the core of the fluid flow and is subject to the influence of all local phenomena, while the other probe is isolated from this influence and serves as an average temperature reference. Two such differential sensors are used in determining the flow rate. The distance between the respective probes of the two sensors that are placed in the core of the flow is known, and the rate of flow of the domains is the ratio of this known distance to the time difference between the corresponding parts of the signals of the two sensors. This time difference is obtained by correlation.
This prior art apparatus has a number of disadvantages, including the fragility of the hot-film probes, and the unsuitability of the apparatus in the measurement of the rates of flow in streams of very different types and of the rates flow of bubbles. With regard to the last mentioned disadvantage, the determination of bubble speed requires that the bubble "bathe" the hot film of the core-immersed probe of the first sensor and subsequently "bathe" the hot film of the corresponding probe of the second sensor. This condition requires that the trajectory of the bubbles must go through the two probes, which is to say that the trajectory must not be disturbed by the presence of the probes. Consequently, this condition implies that the probes should be well separated from one another. The second condition is that the two phenomena must be correlated, which implies that the two probes should be very near one another. The compromise between these conditions is believed to be extremely delicate.
Electric probes having very thin and sensitive tips are used in the apparatus disclosed in U.S. Pat. No. 3,403,554, issued Oct. 1, 1968 to Chevalier et al. The operation of this apparatus also requires the direct contact of the bubbles with each of the two probes, and hence exhibits the associated disadvantages.
Another approach taken in the prior art is exemplified in an article entitled "Correlation Techniques Achieve Valid Flow Measurement Results" appearing in Electronics Weekly, Sept. 23, 1970. As disclosed therein, a thermocouple, one junction of which is in the flow, is used for each transducer. This approach shares certain of the disadvantages of the apparatus disclosed in French Pat. No. 2,161,834; in particular, the unsuitability of the apparatus in the measurement of rates of flow of streams of very different types and of the rates of flow of bubbles.
A differential pressure sensor is included in a known system which purportedly obtains an indication of the percentage of oil and/or water according to weight and volume in an oil-water mixture circulating in a pipeline, as disclosed in U.S. Pat. No. 4,059,744, issued Nov. 22, 1977 to Elderton. The sensor is used conventionally for deriving density, however.
As is therefore apparent, the prior art has not been able to achieve the determination in one apparatus of the rates of flow of turbulent single-phase fluids and of domains of phases contained in polyphase fluids within a wide range of flow types (e.g., extending from bubble flow to plug flow to slug flow in liquid-gas combinations). Furthermore, prior art apparatus generally have not been capable of utilization under very severe operating conditions such as those encountered in producing oil wells, and have not been applicable to the measurement of the speed of bubbles or small domains flowing in a polyphase fluid.